Aegis | Prosthetic Socket Design

A new category of silicone prosthetic socket technology – By Lou Haberman, CPO


The issues of comfort, function, and suspension are always of primary importance with the provision of any orthopedic device. These concerns are also relevant to silicone socket interface technology, which plays and increasingly active role in prosthetics. More and more prosthetists are incorporating silicone liners into their practice because they enhance comfort for their patients, unlike the conventional stump stock. In addition, they provide an efficient means of suspension when used with a shuttle.

As with many new developments, silicone socket technology introduces its own set of issues. When applied to the residual limb, silicone liners introduce compression, which creates a force system independent of, and often at odds with, the prosthetic socket. Unfortunately, many fruitless socket adjustments are performed due to an inappropriately designed silicone liner rather than a deficiency in socket design. Silicone liners should not introduce high or other undesirable forces to the limb. The prosthetic fitting process is already complex; it should not be further complicated by a silicone liner that introduces inappropriate pressures and restrictions which could result in:

  • Discomfort in the popliteal, hamstring, and patella regions
  • Restricted knee flexion
  • Inappropriate anatomical tapering and overall shape
  • Very high pressures developed at various anatomical points

Aegis was created in response to these problems. Aegis is preflexed at 45° and anatomically tapered. This simple yet very significant departure from the current straight, windsock shaped liner designs provides the following benefits:

  • Comfortable sitting at 90° knee flexion for extended periods
  • Minimized wrinkling in the popliteal region
  • Safe and far lower internal liner pressures in all ranges of motion
  • Reduced patella discomfort and pressure
  • Increased range of motion at the knee
  • Absence of knee extension moment when knee is flexed

The design, shape, and processing of Aegis has created a new class of silicone liner. Clinical assessments (i.e. beta testing) of Aegis against straight, windsock shaped liners indicate significant patient preference for it (47 of 51 users, 3 unreported). Of interest, Aegis is also preferred by transtibial amputees using custom made liners.

In addition, scientists and engineers at the Massachusetts Institute of Technology were engaged to substantiate these clinical observations. To our knowledge, this is the first scientific testing of silicone liner technology. To define comfort, internal liner pressures were tested at knee positions of 0°, 45° and 90° (see Figure 1). This preliminary testing was performed statically and in real time. Each test was performed three times and the average of the results was calculated.

The tests determined that Aegis was far more comfortable than other liners because the pressures it generated on the lim were dramatically lower throughout the range of motion of the knee. The “0” or start position for Aegis is 45°. The “0” or start position for straight liners is 0° or full knee extension. In the Aegis liners, pressures were actually lower and more uniform at 0° than in the straight liners. This is due to Aegis’ oval shaped knee region, its anatomical taper, and flexion angle. At 45° and 90° of knee flexion, the differences between Aegis and other liners becomes even more striking.

The importance of sitting comfort cannot be overstated. Many amputees sit for longer periods than they stand or walk. Our testing of liner pressures at the hamstring tendons, patella, and other regions substantiates amputees’ claims of significantly improved comfort. Indeed, some of the pressures (mmHg) generated by straight liners far exceed what are considered to be safe values for prolonged human contact.

Also tested was the compliance of the prosthesis and its silicone liner, that is, movement under load/unload. It is important to understand the functional effect of a fabric matrix incorporated into a silicone liner, and if Aegis’s lack of a fabric matrix would have a deleterious effect on negative compliance, or pistoning. For these series of tests, a comparison was made between Aegis, the ALPS suction socket liner, and the Iceross ™ suction socket with Matrix. Positive compliance (loading, penetration into the socket) and negative compliance (unloading, distal displacement of socket during swing) were measured. This testing assumes the prosthesis is the PTB type and utilizes a shuttle lock type suspension only, with no auxiliary suspension. The margin of error was 1-2mm.

Negative compliance was tested at -10, -10, and -30 pounds of pull. It was calculated that normal prosthetic usage can develop between 10 to 20 pounds of pull during swing phase. These texts indicated that all three liners performed similarly regarding negative and positive compliance. It appears that a fabric matrix does not reduce compliance at all, although it is a advertised to do just that. The data confirmed our initial hypothesis that compliance occurs as a consequence of tissue mobility and cannot be reduced by a single layer of fabric.

Tests also indicate that the addition of a fabric matrix increases liner stiffness and subsequently reduces liner compliance (adhesion) to the limb. This can result in a loss of liner to tissue contact in various ranges of motion. Loss of skin adhesion to the liner can cause air pockets, friction, blistering, and skin irritation and breakdown.

The only effective means to reduce negative compliance (pistoning) is to roughen the inner surface of the socket or provide a proximal auxiliary suspension. As a consequence of these preliminary tests we made the following conclusions:

  • A fabric matrix is irrelevant
  • A fabric matrix reduces skin to liner contact
  • Roughening the inner socket surface reduces both negative and positive compliance

The development of a silicone liner that is preflexed and tapered required considerable engineering time and team expertise. Residual limb length, which until now was considered unimportant, is a required measurement when ordering an Aegis silicone liner. Liner lengths of 4,6,8 and 10 inches are available in all circumferential sizes. Limb length is determined by measuring from the midpatella tendon to the distal end.

Circumferential sizes range from 12 through 30 cm (even numbers). Presently, only liner sizes 18 cm through 26 cm are available. The remaining sizes will become available in the next few months. The sizing chart (above) anticipates the appropriately sized liner required for proper limb compression and fit. Aegis liners are available with or without a shuttle umbrella.

The Aegis transtibial prefabricated silicone liner is unique in design and without peer. Aegis responds to the concerns of prosthetists and the needs of their patients with its innovative design and anatomical considerations.

Lou Haberman, CPO

is the R&D Director of Engineered Silicone Products and the inventor of the Aegis Silicone Liner. In August this year, Seattle Limb Systems formed a strategic alliance with Engineered Silicone Products, or ESP. This newly formed company has redefined the use of silicone as a prosthetic interface with its invention of Aegis. With its anatomically correct shape, Aegis creates a new high standard of clinical care for amputees.